Semiconductor thin films are widely used for thin film transistors (TFTs) and solar cells. In particular, polycrystalline silicon TFTs that use polycrystalline silicon (polySi) have higher carrier mobility than amorphous silicon TFTs that use amorphous silicon. Therefore, the polycrystalline silicon TFTs are widely used as a liquid crystal display device, a liquid crystal projector, a switching element for organic EL display devices that use organic electroluminescence elements, or a circuit element of driving drivers of these display devices.
A so-called low-temperature polysilicon process that uses only a low-temperature process performed at about 600° C. or less has been developed as a manufacturing technology of such a polycrystalline silicon TFT, which reduces the costs of a substrate. In the low-temperature process, pulsed laser crystallization technology in which a silicon film is crystallized using a pulsed laser whose oscillation time is extremely short is widely used. Pulsed laser crystallization is a technology that uses a property in which a silicon thin film on a substrate instantly melted by applying high-power pulsed laser beams thereto crystallizes during its solidification.
Furthermore, a method for crystallizing a lower-layer semiconductor thin film by applying laser beams from the upper side of a light absorption layer has been proposed in recent years. In this case, for example, an amorphous silicon film is formed, and a first insulating film composed of silicon oxide or silicon nitride is formed thereon. Subsequently, a light absorption layer composed of a metallic material is formed, and a second insulating film composed of silicon oxide or silicon nitride is further formed. After that, laser beams are applied to the laminate while being relatively moved. As a result, the temperature is rapidly increased at the area on the light absorption layer where the laser beams are applied, and a high-temperature region is formed in the amorphous silicon film due to thermal conduction. The amorphous silicon film is melted and crystal growth occurs. Thus, a polycrystalline silicon layer whose crystal grows from a liquid phase is formed in the area where the laser beams have been scanned. Note that the second insulating film is formed for inhibiting oxidation of the light absorption layer caused by temperature increase. Therefore, when the laser beams are applied in an inert gas atmosphere, the second insulating film is not required (up to this point, refer to, for instance, Japanese Unexamined Patent Application Publication No. 2004-134577 Paragraphs 0035 to 0048).